This invention relates to a method for recovering chemicals from flue gases obtained from burning in pulp mills which use sodium as cooking base and from a water solution of the smelt i.e. green liquor. Waste liquor containing remarkable amounts of sodium chemicals, the recovery of which is economically profitable, is obtained as a waste from the pulping performed by sodium based sulphite cooking liquor. For the recovery of chemicals, waste liquor has been concentrated by evaporating and burning it in the recovery boiler whereby flue gases containing sulphur and carbon dioxides are obtained, and a smelt, from which so-called green liquor containing sodium sulphide has been obtained by dissolving it into water. Sulphur dioxide is washed from the flue gases by a sodium carbonate solution made from green liquor. Sodium sulphite formed then is used as such in the sodium suphite pulping.
The before-mentioned sodium carbonate solution is produced from green liquor by precarbonating green liquor first by means of the washed and cooled flue gases whereby sodium sulphide reacts with the carbon dioxide of the flue gases and sodium bisulphide is obtained.
The precarbonated sodium bisulphide solution is thereafter brought to react with the sodium bicarbonate solution produced in the process, whereby sodium carbonate and hydrogen sulphide are obtained. Hydrogen sulphide has been stripped by steam and burnt in a sulphur burner into sulphur dioxide which has been led together with other flue gases into the washer for production of sodium sulphite.
By leading the precarbonated sodium bisulphide solution according to the Finnish patent specification 54946 in the stripping stage into contact with the solution which contains at least a double amount of sodium bicarbonate compared to the equivalent amount of sodium bisulphide, essentially all sodium bisulphide can have been converted into sodium carbonate and hydrogen sulphide. Then essentially all washed and cooled flue gases, except those ones used for the precarbonation, have been led into the carbonation stage, into which a part of the stripped solution containing sodium carbonate has been brought back for raising its sodium bicarbonate concentration. Because the solution leaving the stripping stage does not contain any sodium bisulphide, it can be fed into the washer directly without any extra crystallizations. Before the process according to the Finnish patent specification 54946 the stripped sodium carbonate could not namely be led directly into the washer of the flue gases because this solution still contained some sodium bisulphide. Therefore the stripped solution had first been taken into a crystallizer in which sodium carbonate and sodium bicarbonate have been crystallized from the solution. These crystals have then again been dissolved in water and part of the solution thus obtained has been led into the flue gas washer in which the sulphur dioxide of the flue gases reacts with sodium carbonate for production of sodium sulphite. The rest of the solution has been led into a carbonation reactor in which the sodium carbonate of the solution has been allowed to react with the washed flue gases to increase the sodium bicarbonate content of the solution so much that this solution could be taken into the stripping stage.
The sodium carbonate solution free enough from sodium bisulphide needed for the flue gas washer can then be produced in two different ways, either in a way according to the Finnish patent specification 54946 by using in the stripping stage at least a double excess amount of sodium bicarbonate compared to a sodium bisulphide equivalent amount or by crystallizing sodium carbonate and bicarbonate crystals from the solution obtained in the stripping stage, by separating the crystals and by dissolving these again and by feeding the solution obtained into the washer.
A method based on the crystallization and the separation of crystals is relatively cumbersome, in addition to which the devices needed are very expensive. The advantage of this method is again the fact that only so big a part of the solution from the stripping stage needs to be crystallized as the sodium carbonate solution is needed for the washer. In this way just that needed amount of the sodium carbonate solution with the lowest possible residual sulphide content, can be produced while the rest of the solution with the higher sulphide content is used for purposes which are not impeded by the higher sulphide content.
The advantage of the method according to the Finnish Pat. No. 54946 is that so pure a sodium carbonate solution regarding sodium bisulphide is obtained directly from the stripping stage that it can be led into the flue gas washer without any extra crystallizations. This makes it possible to use simplier devices, whose space requirements are lesser and investment costs lower. The disadvantage of this method is that the flue gas washer cannot receive all sodium carbonate leaving the stripping stage so that part of it has to be used elsewhere and in such objects which do not provide so pure a solution regarding sodium bisulphide. Thus too much steam or heat and bicarbonate have to be used in this method.
The object of the present invention is thus to provide a method for recovering sodium chemicals from the flue gases and green liquor obtained from burning in the pulp mills so that sodium carbonate and sodium bicarbonate need not to be crystallized from the solution obtained in the stripping stage, these crystals need not to be separated and dissolved again for feeding into the flue gas washer and however, so that just as much sodium carbonate solution, which is pure enough regarding sodium sulphide, is obtained from the stripping stage as is necessary in the flue gas washer whereby the rest of the solution can be used elsewhere, for processes in which the higher sodium bisulphide content in the sodium carbonate solution can be allowed. The purpose of the invention is thus to accomplish a more economical and flexible method for converting sodium sulphide and/or sodium bisulphide into other products using less steam or heat and without need to feed as much sodium bicarbonate solution as in the prior art into the stripping stage.